Distribution of permafrost and spatial variations of its mean annual temperature

The great extent of the territories of the former USSR from west to east and from north to south, the complexity of orographic, geological, tectonic and hydrogeological structures, of landscape and of climate, together with the changes in the late Cenozoic, are responsible for the variety of geocryological conditions in their different parts.

The zonal distribution of permafrost in the territory of the former USSR is associated with two main circumstances:

1) the history of the cryogenic development of the upper layers of the Earth's crust in Neogene-Quaternary time during which the stage was set for the present geocryological conditions;

2) the present conditions of heat exchange at the surface and in the ground. The mean annual ground temperature for the particular period, including for short-period fluctuations, indicates the level of heat-exchange for every climatic sequence.

The present heat-exchange conditions under which the mean annual ground temperatures are being formed, are defined by the following basic factors and conditions:

1) the dependence on latitude of the incoming solar radiation, upon which depends the radiation balance at the Earth's surface;

2) the effect of the Atlantic, Arctic and Pacific oceans on the atmospheric circulation, which gives the regional nature of climate;

3) the distance between the territory and sea basins and the position of the territory within the continent, this being responsible for the climatic continentality, i.e. for the amplitude of the annual fluctuation of temperature of the air and of the Earth's surface;

4) the orographic conditions which control the normal distribution of heat exchange with altitude in middle and high mountain regions and the associated inverse distribution within mountain plateaus;

5) the geological-tectonic conditions, with which ground composition, structure and properties, and the value of the geothermal flux and gradient of temperature in the permafrost are associated;

6) the hydrogeological conditions existing as a result of interaction of the permafrost and groundwater;

The combination of these factors and conditions results in great variability in distribution and mutual arrangement of the perennially frozen and unfrozen ground and in the ground temperature regime. The distribution of permafrost, the spatial relationship between the permafrost and unfrozen ground, as well as the spatial changes of the mean annual ground temperature, show a close connection with the landscape-climate zones. Therefore we should study the geocryological environment of large territories in relation to the associated natural and geocryological regions. Within each region the pattern of the permafrost distribution and temperature regime is similar and differs from that of the adjoining regions. Such a regional or zonal approach allows study and comparison of the main geocryological trends within the whole territory of the former USSR.

We can appreciate visually the regular change of the mean annual ground temperature from south northward and from west eastward and with altitude in mountain regions, from the Geocryological Maps of the former USSR at the scale of general survey (1:2 500000 - 1:25 000000). In these the

geotemperature zones are shown with temperatures to an accuracy of 1 to 2°C. We can see from the map (Fig. 15.1) the essential south-eastward departure of the identical geotemperature zones which are situated 2-3° further south in Western Siberia and as much as 6-8 "further south on Prilenskoye plateau and in Southern Yakutiya as compared with the regions within the European part of the former USSR, as a result of regional climate and the continentality of the heat exchange at the Earth's surface. At the present time the distribution of the permafrost and its temperature regime within the former USSR are such that the severity of geocryological conditions increases from south northward and north-eastward and with altitude in mountain regions. An azonal heat exchange appears locally superimposed on this background, with a combination of regional natural factors playing a greater role than that of latitude. The valleys of large rivers such as the Ob, Yenisey, Lena and others situated meridionally, and large lakeSj do not freeze to the bottom and large sand masses of eolian, fluvioglacial, alluvial and other origin are geocryologically azonal. On the former USSR territory as a whole the mean annual ground temperatures vary over a wide range from +21°C in unfrozen ground of the subtropical zone to —15°C and lower in the permafrost of the Arctic zone and in high mountains. All of the territory of the former USSR is divided into two main regions: of permafrost with seasonal thawing from the surface (the permafrost region) and of soil with seasonal freezing of soil horizons and of the ground below (seasonal freezing region). The conventional dividing line separating these regions drawn around the southern permafrost islands in general maps is termed the southern limit of the permafrost region. To the north of the southern limit (and in mountains higher than where it is situated) the area of the permafrost islands and their numbers increase and the mean annual ground temperatures are lower. In connection with the latitudinal variation of the solar radiation and under the effect of the Atlantic warmth, the permafrost limit is generally directed from the northwest (Kola Peninsula) south-eastward (up to Yenisei river), being turned south-westward and westward in the neighbourhood of the city of Krasnoyarsk and then passing through the foothills of the mountain system of the southern former USSR.

The relationships of unfrozen to frozen ground and the spatial changes are closely associated with and depend on the ground temperature regime. The most southern permafrost islands are a few ten to a few hundred square metres in area, and can develop periodically in the unfrozen ground within two geotemperature zones adjacent to the permafrost region. In these zones the ground is characterized by rather low (from +3 to 0°C) positive mean annual temperatures. Within them permafrost can occur under undisturbed conditions during cooling, but only in peatlands and in clayey-silt deposits which are free of snow cover or heavily shaded. In areas of geotechnical disturbance (rock dumps, quarries etc.) permafrost may develop even without any cooling. Within the permafrost region the southern and northern geocryological zones are distinguished depending on the distribution of permafrost and its peculiarities, mean annual temperatures and thickness.

The southern geocryological zone

This zone is characterized by the discontinuous distribution of the permafrost, and its aerial extent and continuity which increase northward from 25% and less in the area of the sporadic permafrost to 25-75% in the areas of massive permafrost and islands and to 75-90% in the areas of the incomplete permafrost (Table 15.1).

In the southern zone the dynamic nature of the permafrost and thawed ground, especially near the southern limit, is associated with mean annual ground temperature fluctuations from year to year of as much as 1-2 °C and even more in years of extreme weather conditions. Such changes can cause new permafrost to form or may cause a separation of the permafrost from the seasonally frozen layer. Investigation of materials and estimations show that such effects occur most often in locations with mean annual temperatures ranging from +2 to — 2°C. The permafrost in the southern geocryological zone is for the most part of late Holocene cryogenic age. New permafrost forms in the regions of intensive change in natural conditions. In addition to frozen islands and masses, thawed ground (radiation-thermal taliks) occurs and is widespread in all topographic elements, under conditions where the summer ground heat balance exceeds the winter one.

The smallest proportion of permafrost to unfrozen ground is typical of the (sub)zone of sporadic permafrost and permafrost islands adjacent to the region of unfrozen and seasonally freezing ground. The prevailing mean annual temperatures in the unfrozen and frozen parts within this subzone range from +2 to —0.5°C (Table 15.1). Under these conditions the permafrost islands exist only under favourable combinations of permafrost-forming factors such as surface shading, full or partial absence of snow cover, high peat content of the deposits, wide development of silty-clay surface-water-confining strata, etc. Thus within the flat areas of this subzone the appearance of the first permafrost islands is associated with peat frost mounds, with shaded peaty clay-rich silt, covered with mosses, and with the bottoms of valleys of streams and watercourses, overgrown with thick coniferous forests. Within the flat watersheds the permafrost can be

Table 15.1. Distribution of the perennially and seasonally frozen (thawed and unfrozen) ground on territory of former USSR

Region (with respect to presence or absence of frozen ground)

Zone

Subzone

Type of seasonal thawing of permafrost and seasonal freezing of ground (with respect to the stability of the process)

with respect to specific features of the perennially frozen, thawed and unfrozen ground distribution